1. Field of the Invention
This invention relates generally to a workpiece micro-positioning apparatus for correcting small pitch and yaw location errors in a workpiece supported on a machine tool fixture for manufacturing operations such as precision machining and assembly.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Fixturing is a critical and expensive process that must be undertaken in preparation for machining a workpiece such as a cylinder head for an internal combustion engine. In all high volume machining systems, dedicated fixtures are used to locate and hold workpieces. The dedicated nature of fixturing is arguably the greatest barrier to implementing truly flexible machining systems for workpieces.
For example, automobile manufacturers are currently faced with consumer demand for a large variety of powertrain options. This demand compels a change in manufacturing from a dedicated transfer system capable of producing 250,000 workpieces of a single configuration annually to a flexible transfer system that can produce 50,000-100,000 each of workpieces of different configurations. However, despite the availability of highly flexible CNC-type machines, the fixturing of workpieces of varying configurations during machining at high production rates requires time-consuming changeovers and incurs a large capital cost. In many fixturing systems, locating surfaces on the fixture and corresponding locating surfaces on each workpiece must be machined to close tolerances to insure that each workpiece is accurately positioned in the fixture for machining. The requirement for fixed, dedicated, mechanical locators for each workpiece configuration is the critical technology that limits the flexibility of machining fixtures.
A part location system must account for three attitude and three location errors that can result from loading and clamping a workpiece in its fixture. As shown in FIG. 1, the directions of the three location errors are along horizontal X and Z axes and a vertical Y axis. The directions of the three attitude errors are pitch about the X axis, roll about the Z axis, and yaw about the Y axis. Machine tools generally provide linear motion along the X, Y and Z axes with the Z axis oriented in the direction of spindle feedout. Four of the errors resulting from loading and clamping, i.e., in the X, Y, and Z directions and roll about the Z axis, can be corrected by simple work coordinate offsets in the machine tool controller. The remaining two errors, pitch about the X axis and yaw about the Y axis, generally within xc2x12.5 degrees, cannot be corrected by such a machine tool.
U.S. Pat. No. 4,262,891 issued Apr. 21, 1981 to Kinney, discloses a workpiece positioning apparatus that includes a tri-axial workpiece holder. The holder includes three rotatable and releasably lockable platforms configured to support a workpiece for rotation about a pitch, roll and yaw axes. The holder also includes three drives operably coupled to the respective platforms to rotate the platforms about their respective axes to change the alignment of a supported workpiece relative to a single tool. The Kinney positioner cannot position workpieces for machining by other tools. Neither can the Kinney positioner operate without internal actuation systems or power, or rotate more than one platform by manipulating only one of the platforms. Also, the stability provided by the Kinney workpiece holder for precision machining operations is limited because Kinney""s first and second platforms are supported on respective drive shafts, each shaft being less than a fifth the diameter of their respective platforms.
U.S. Pat. No. 5,239,160 issued Aug. 24, 1993 to Sakura et al., discloses a workpiece positioning apparatus for a laser cutter. The apparatus includes a three-axis table mechanism including an x-axis table supported for linear motion in the x direction. The x-axis table is supported in overlapping relationship on a y-axis table that is supported for linear motion in the y direction on a z-axis table. The z-axis table is supported on a support surface for linear motion in the vertical z direction. A biaxial workpiece holder is supported on the three-axis table mechanism and tilts and rotates a workpiece about the x and z axes, respectively. The holder comprises a rotatable work holder platform that supports a workpiece for rotation about a first (vertical z) axis, and a second platform configured to support the first platform for rotation about a second (x) axis normal to the first axis. The workpiece positioning apparatus of the Sakura et al. patent includes linear x, y and z axis drives operably connected to the respective x, y and z-axis tables. The drives move the tables in the respective x, y and z directions. A first servomotor is drivingly connected to the rotatable work holder platform and rotates the work holder platform about z-axis. A second servo motor is drivingly connected to the second platform and rotates the second platform about the x axis. However, the Sakura et al. workpiece holder is fixed to the table mechanism and cannot positively locate a workpiece on locating platforms of different machine tools or laser cutters. Neither can the Sakura et al. system operate without internal actuation systems or power or to rotate more than one platform by manipulating only one of the platforms. Also, the stability provided by the Sakura workpiece holder, while sufficient for laser cutting, would be inadequate for precision mechanical machining operations. One reason for this is that Sakura""s first (work holder) platform is supported on a drive shaft less than a third the diameter of the platform and its second platform is cantilevered on another drive shaft.
The invention is a workpiece micro-positioning apparatus for correcting small pitch and yaw location errors in a workpiece supported on a machine tool fixture for manufacturing operations such as precision machining and assembly. The apparatus comprises a biaxial micro-positioner including a pitch motion mechanism configured to support a workpiece for rotation about a pitch axis and a yaw motion mechanism supporting the pitch motion mechanism for rotation about a yaw axis generally normal to the pitch axis. A base supports the pitch and yaw motion mechanisms. The workpiece micro-positioning apparatus also includes a fixture fixed to the micro-positioner and configured to releasably hold a workpiece. The micro-positioner is configured to allow rotation of the fixture about the pitch and yaw axes to correct any pitch and yaw deviations from a desired workpiece attitude relative to the base and to releasably lock the pitch and yaw motion mechanisms following such correction. The workpiece micro-positioning apparatus is used in conjunction with a first machine tool releasably supporting the micro-positioner and configured to rotate the micro-positioner, fixture and a workpiece mounted in the fixture, about a roll axis generally normal to the pitch and yaw axes as required to correct any roll error in workpiece attitude. The first machine tool is configured to then perform a first machining operation on the workpiece. The base precisely and releasably locates the micropositioner on the first machine tool.
According to another aspect of the invention, the apparatus includes a manipulator configured to releasably engage the micro-positioner and rotate the pitch and yaw motion mechanisms about the pitch and yaw axes, respectively.
According to another aspect of the invention, the base is configured to positively locate the micro-positioner, fixture, and workpiece on one or more machine tools. Each machine tool is configured to move the micro-positioner into a position positively locating a workpiece mounted on the fixture for machining.
According to yet another aspect of the invention, the yaw motion mechanism includes a rotatable and releasably lockable rotary plate supported on the base for rotation about the yaw axis. The pitch motion mechanism includes a releasably lockable tilt plate disposed parallel to the pitch axis and supported on the rotary plate for tilting motion about the pitch axis.
According to another aspect of the invention, inner and outer radially spaced concentric supports rotatably support the rotary plate of the micro-positioner to limit motion of a supported workpiece under machining loads.
The invention also includes a method for correcting small pitch and yaw location errors in a workpiece supported on a machine tool fixture for manufacturing operations such as precision machining and assembly. According to this method one can correct small pitch and yaw location errors in a workpiece supported on a machine tool fixture by first securing a workpiece on the fixture, then causing the manipulator to engage the micro-positioner. The manipulator then corrects pitch and yaw attitude errors of the workpiece relative to the micro-positioner base by actuating the manipulator to rotate the workpiece about the pitch and yaw axes. The manipulator then disengages the micro-positioner. A machine tool is provided to perform a machining operation on a workpiece supported on a fixture supported on a locating platform on the first machine tool. The micro-positioner is precisely located in a predetermined position on the first machine tool.
Objects, features and advantages of this invention include providing a workpiece micro-positioning apparatus that corrects the two misalignments of the workpiece that cannot be corrected by a standard 3-axis machine tool, that allows the fixture and workpiece to be moved as a unit from one machine tool or three-axis machine tool to another without repeating the micro-positioning procedure, that can take a rough workpiece and precisely orient it just once for a series of manufacturing operations, that can operate without internal actuation systems or power, that is sufficiently stable to support precision mechanical machining operations, that drastically reduces the lead-time necessary for workpiece changeover and the overall cost of manufacturing, and that eliminates the need to fabricate mechanical locators for each workpiece to be machined.